Minor suggestions for input validators

[liancheng]

I was playing with Trogon and tried to add text input validators in Textualize/trogon#59. Some thoughts:

1. The Integer validator should reject integer values in float point format.

   Today, the Integer validator accepts input like 1.0, which is rejected by click.INT:

   import click
   
   
   @click.command()
   @click.option("-v", type=click.INT)
   def main(v: int):
       pass
   
   
   if __name__ == "__main__":
       main()

   $ python test.py -v 10.0
   Usage: test.py [OPTIONS]
   Try 'test.py --help' for help.
   
   Error: Invalid value for '-v': '10.0' is not a valid integer.
   

2. The Function validator should accept a function returning a str | None instead of a bool to indicate the failure description.

   I was trying to call ParamType.convert in a Function validator to validate input. Ideally, I should be able to capture the error message in the BadParameter exception to generate the failure description. But right now the failure description can only be a fixed string.